And harold s



May 8, 1928.

` R. BOWN ET AL.

RADIO TRANSMISSION SYSTEM 2 Sheets-Sheet l -UWM w IF. wy ilklxlwl, l l l IIT @im j L. T@ v hx d# N) uw x:

ATTORNEY May 8, 1928.

' R. BowN ET AL.

RADIO TRANSMISS ION SYSTEM ATTORNEY 2 Sheets-Sheet 2 Filed Dec. 18. 1924 Patented May 8, 1,928.

YUNITED, STATES 1,668,757 `'PATENT OFFICE.

RALPH BO'WN,0F MAPLEWOOD, NEW JERSEY,l .ANDI HAROLD S. HAMILTON,`OF NEW YORK, N. Y., ASSIGNORS TOv AMERICAN TELEPHONE AND TELEGRAPH COMPANY,

A CORPORATION OF NEW YORK.

RADIO TRANSMISSION SYSTEM.

Application led December 18,-1924. lSerial No. 756,803.

This invention relates to radio transmission systems and more particularly to arrangements for improving the operation of radio circuits used for long distance transmission.

In order to operate a radio telephone transi-nission system on a two-way basis where the intercommunicating stations are widely separated, as, for example, in trans-Atlantic communication, many problems arise ,which are not met with in the' usual radio practice. For example, it is necessary that some part of the system, frequently including the high frequency part of the system, be operated on a four-wire basis and connected to terminal two-wire connections. A circuit of this type has a tendency to be unstable due to poor hybrid coil balances or to variations in the transmission equivalent of the high frequency part of the circuit. This ditculty may be overcome, as herein disclosed, by associating echo Suppressors with the four-Wire part of the system, these elements operating to 'disable the circuit for transmission in one direction in response to the transmission of currents in the opposite direction. Preferably., also, the `echo suppresser units may be arranged to normally disable the circuit to the radio transmitter to prevent local singing.

In accordance with the present invention it is further proposed to provide. means to enable the circuit to be operated not only when the hybrid coil balances or the radio circuit transmission equivalents are at unsuitable values for ordinary operation, but also when the radio loss from the local tranemitter into its associated local receiver is insufficient to prevent singing or poor quality. The last mentioned condition becomes particularly pronounced when the system is designed to transmit in both directions at the same radio frequency, and this is almost necessary ina trans-oceanic radio telephone system in order to conserve the frequency range. Under such conditions the tendency of the local circuit to sing, due to the transfer of energy around the circuit from the local radio transmitter to the local radio receiver, may be overcome by providing a pair of echo suppressor units at each of the two widely separated communicating points. Each local four-wire circuit will then have .its IIlSmltr V110131.1115 disabled to prevent singing and the echo suppressor units will respond to voice `currentswto disable either one of the paths of the four-Wire part of the llocal circuit,`while enabling the other path ,to carry transmission, dependingr on the direction in which signaling is taking place.

The invention may now be more fully understood from the following description when read in connection wth the accompanying drawing, Figures l and 2 of which, .when placed end to end, illustrate the circuitswhereby two widely separated points may be put into two-way communication by radio telephony.v

Referring to the drawing, L designates an ordinary telephone line entering a telephone exchange at some point. The line L terminates in the usual hybridcoil l() and balancing network N, the hybrid coil 10 serving to connect the line to a. transmission path LE for transmission from west to east and a transmission path LNV for transmission from cast to west. The transmission path LE includes suitableuamplifiers or repeaters such as REl, RE,` and extends to a suitable location for a radio transmitter comprising a radiating antennaATand radio transmitting apparatus TT, schematically indicated in the drawing. Similarly, the transmission kpath LlV includes suitable one-way amplifiers or repeaters RW4 and RW3 and extends to a radio receiving apparatus, schematically indicated at RR, and located at some suitable point which is` not too close to the transmitting station.

In order to enable the system to transmit only in one direction at a time and in order to preventl sin ging or noisy conditions of the four-wire part of the circuit, an'echo suppressor' unit may bc located at any point, as, tor example, an intermediate repeater point. In the case illustrated the echo suppressor combination is located at the repeater station including the repeaters RE, and RVVT The vecho suppressor.comprises two separate units or elements, one for controlling transmission in lone direction and the other for cont-rolling transmission in the other direction. For example, the element associated with the transmission path LE comprises an amplifier-detector arrangement VE of a type Well known in the art. Similarly, the unit associated with the path LW comprises a similar amplifier-detector arrangement VW. The

i LW and the hybrid coil to the path LE to set up singing or noisy conditions of the circuit." The relay operates in'response to rectified voice currents transmitted over the circuitl LE to close the contact 2 lto sh0rtcircuit the path LlV and therefore disable thevcircuit for transmission from east to west.v At the same time the contact 3 is opened to render the path LE operative for transmission from west to east; A delay network X is inserted between the point at which the amplifier-detector unit VE is connected and the short-circuit connections of the contact 3 to enable the contact 3 to be opened before the voice Currents arrive at the point where the short circuit is applied.

In order to reduce the transmission from the transmittingV antenna AT to the local radio receiver RR, particularly Where transmission and reception both take place at the` same wave length, a wave antenna AR may be employed, Vwhich has a directive effect. The wave antenna, as illustrated, comprises a pair oi conductors 11 and 12 extendedvhorizontally at a suitable point above the ground to a distance of one yor more wave lengths. These conductors act as an antenna upon whichV the wave coming from the. right, for exan'iple,'buildsup by successive increments so that it reaches its maximum amplitude at the antenna terminal at theleft. The waves picked up by the two conductors 11 and 12 of the antenna are transmitted through a ground connection 13, including the primary winding of a transformer 14. In order that the wavesy thus received may ail'ect the= receiving,1 instrument connected to the opposite end'of the antenna conductors a secondary Vwindingr 15 is inductively associated with the winding" 14 to connect the two conductors 11 and 12 in series. As a result the waves built up on the antenna conductors 11 and 12 in parallel are transmittcdlback over the conductors 11 and 12, connected in series with each other soy that they form a line circuit. The waves'atthe receivingrend are then passed through a transformer 16v and applied to the radio receiver RR through a suitable amplifier 17.

Assuming that transmission `and reception take place on ditterent wave lengths and that theenergy transmitted from the transmitting' antenna AT to the receiving apparatus RR is so small, by reason of the selectivity of the circuit and the directive reception of the antenna, that singing will not take place locally from -the antennal AT .to the radio .receiver RR and -thenceiaronnd through the hybrid coil 10fthe` systemv will operate for two-way communication with a similar circuit at a distant point without requiring the provision of a second pair of echo suppressor units 'for `the circuit` at the distant point.

The contact, which is normally closed to short-circuit the conductor LE, prevents singing about the complete four-Wire path f through the ether to the distant receiving apparatus, then transmitted at low fre-r i quencies over the. West to east transmission path to thel hybrid coil at the distant point, thence through said hybrid coil over the east to west pathlto the transmitterat the distant Aim j point, kfrom whence'the currents are radiated at another radio frequency,` received bythe wave antenna AR, translated to voice frequencies by the receiver RR and transmitted over the path LlV and throughv the hybrid coilf10 and thence over the path LE, will be.

prevented from being transmitted to the radio transmitter TT and again radiated by reason of thefshort circuit at the contact 3. Such'circulatory transmission, in short, cannot take place in the normal condition `of the circuit and it is not Vnecessary to vprovide a secondpair of echo suppressor units at the distant station to prevent-'such circulatory transmission. n

If a subscriber at one point speaks lto Va subscriber at the distant point `the voice currents incoming from the line L Will pass through the hybrid coil 10 to the path LE andV afterbeinr amplified by the repeaters RE1L and RE2 will be impressed on the delay 1' network X. Part of the energy is diverted,

however, into the amplifier-detector unit-VE and operates the relay'to close the Contact 2 and open the contact. The' closure of contact'2 disables the path LW vso that transmission from thewdistant stationcannot be received. Contact 3 in the meantime is opened before the voice wave passes through the delay network X so that the path from f west to east is now operativey through to the distant receiving point. The wave then passes through the filter TF Vto' the kradio transmitter TT and is translated to a radio frequency for transmission fromV the an-V tenna AT. The subscriber vat the distant end cannot break in to talk in the reverse direction until the subscriber who is 'now talking interrupts his conversation.l

iVhen the subscriber at the distant point talks the voice currents are transmitted over a similar transmission circuit from east to west and are radiated from the transmitting station at the distant point at a different Wave length from that employed for transmission from west to east.' Upon being received by the Wave antenna AR the Waves are impressed through the amplifier 17 upon the radio receiver RR. The voice Waves are detected by the receiver and transmitted through the filter RF to the east to west path LW and after being amplified by the amplifier RV3 are transmitted on to the second amplifier RgiV. A part of the energy-` is diverted into `the amplifier-detector unit V\V to close the `contact 1. The contact 2 is, of course, normally open and contact 3 is closed. Y short-circuits the path LE and also prevents thetransmission of energy from the path LE to the amplifier-detector unit VE. This operation takes place before .the voice wave passing from the output of thev repeater RW3 has had time to pass through the hybrid coil 10 and back over the path LE to the delay network, When the voice currents have. passed to this point they will be blocked by the short circuit established at the contact 1. The ytransmission path from West to east is therefore completely blocked but transmission may take place freely from east to West. The received voice currents passed through the repeater RV, are, of

f course, transmitted overthe line L to the subscriber.

lVhere transmission in both directions takes place at the same Wave length, as must frequently be the case in order to` conserve space in the frequency spectrum, the provision of a pair of echo suppressor units in connection withk one of ythe four-Wire circuits at one of the intercommunicating points is not sufficient to enable the system to operate. The amount of energy radiated from the local transmitting antenna to the local receiving antenna (even Where lthe receiving antenna is balanced by directivity or other means with respect to thetransmitting antenna) is very great and singing .may take place from the transmitting antennato the local receiving antenna at the distant point and over the four-Wire local circuit at the distant point because no echo suppression apparatus is provided there. Thile thisv singing is prevented in connection with vthe circuit of F1g. 1, the singing at the dlstant intel-communieating point will prevent theA use of the circuit for two-Way communication. In `order to provide for this difficulty the circuit at the distant point, which is illustrated in Fig. 2, must also be provided with echo suppression units. The apparatus of the circuit of Fig. 2 is now in all respects similar tojthat shown-in Fig. 1, the

The operation of the contact 1 corresponding parts being designated by the same reference characters primed. It Will be noted that in Fig. 2 the contact 3', controlled by the echo suppressor unit VW', is normally closed, thereby disabling the transmission path LVV and preventing singing due to the transmission of energy from the radio transmitter TT' to the radio receiver RR and thence over the path vLE through the hybrid coil 10 and back vover the path LV. Similarly, the normally closed circuit 3 of Fig. 1 prevents singing over the local four-Wire circuit at the other end. The contacts 3 and 3 therefore perform two functions. They prevent singing over the local four-Wire circuits at the two distant ends of the system and they furthermore prevent circulation of lenergy from the hybridV coil 10 over the complete channel from West to east to the hybrid coil 10 and thence back over the complete channel from east to West to the hybrid coil 10.

Tf a subscriber at the west station of the system desires to talk to a subscriber at the east station, the voice currents passing over the circuit LE will operate the amplifier-detector unit VE as before to close the shortcircuitat 2 and open the short-circuit at 3. The Vpath LE is now operative and the path LTV is inoperative. i Local singin" still cannot take place for one ofthe paths (the path LW) is inoperative. The voice currents operate the radio transmitter TT to cause transmission at the chosen Wave length from the antenna AT to the distant receiving wave antenna AR. This receiving antenna is similar to that described in connection with Fig. 1 and'operatcs in a similar manner to pick up the Waves over the two sides of the ,circuit in parallel and transfer them back over the conductor Wires in series as a transn'i-ission line, the waves then being passed through an amplifier 17 to the radio receiver RR. Upon being detected, the voice currents are transmitted over the path LE and through the repeater RER. The amplified voice currents now operate the amplifier-detector unit V l. to close the short-circuit 1. The path LNV is already disabled at a short circuit controlled by the contact 3/ but the closing of the contact 1 prevents any possibility of the voice-operated unit VW" responding to echo currents or to transmission from the sulinscriber at the east station duringrthe time the voice, waves are traveling from the west station. The voice waves pass on through the repeater RE, and thence into the line L. It will be obvious that as longl as the voice waves are being transmitted thepcircuit is leo ' fore `p'ass'through the path LW" to` be reradiated by the transmitter TT',

Then the transmission of voice `currents from West to east ceases the subscriber at the east station can talk in the opposite direction, in which case the echo suppressor units VW andVlV Will be operated succes,- sively to disable'the circuit from West to east and open the circuit through from east to west in a manner vsimilar to that already described.y Singing around the entire circuit 'including the east and westv lterminals is impossible when echo suppressor-s are provided at both ends for" the same reasons that apply when the echo suppressor arrangements are placed,at oneend only ot the circuit. Singing around 'the local circuitof one end from transmitter to receiver is lnot possible since an impulse startingr on either side of the circuit au-ton'iatically cuts oil' the other side.

Another dificultywhich arises in the 0peration of circuits such as those above described is due to the fact that the transmission Vloss from the local radio transmitter Vinto the local radio receiver -isV sometimes small enough sov that noise currents radiated from the transmitter are suilicient to pass through the receiverfand operate one of the echo suppressor units. For example, in Fig.

l, it these noise currents pickedvup by the radio receiver RR should operate the echo suppressor unit VW the echo suppressor unit VE would be'short-circuited-V vSimilarly, in the circuit of Fig. 2 the operation of the echo suppressor unit VE would short-circuit the echo suppressor unit VW. If the subscriber at the West station attempted to talk the lsystem Would be inoperative or the voice currents Wouldnot actuate the echo suppressor u nit VE'toopen up the pathr from West vto east- Similarly, if the subscriber at the east station' undertook to talk the system wouldbe inoperative in this direction 'also' because the voice currents would not actuate the echo suppressor unit VV to render Vthe circuit operative from east to West. Y

Y For this reason it is desirable vthat there be Va certain amount of discrimination between the transmitting and receiving channels at the radio receiver. Where the same wave length is employed for transmission in both directions, however, selectivity cannot be taken advantage of in order to attain the desired discrimination and reliance must be placed on directional and balancing methods to obtain sutlicient discrimination to avoid noise, the two sets of voice-operated devices, as described above, serving to prevent singing, if the noise is kept down suliiciently to prevent locking' up the circuit in the manner just described,

tially unidirectional, that is, it Will receive Waves transmitted from. east 'to West but will 'be substantially unresponsive to Waves transmitted in the opposite direction. Such an antenna, however, is not completely unidirectional and will there-fore respond to a small extent to Waves transmitted from the local transmitting 'antenna AT, althoughlit may be located `in 'a directionfquite different from the distant transmitting antenna AT'. In order tobalance out the component corresponding ftothe enert, transmitted from the antenna AT, a localiy loop antenna AB may be provided, this loop beingrotatable in a Well, known manner so that it Will yhe arranged to pick up 7energy from vthe tra-nsmitting antenna, AT buttopic'k upY little Vor no energy Afrom the distant transmitting antenna A'l". The directive loop-"AB is through an adjustable phase vchanger 22 and potentiometer 23 yto a vacuum tubeamplife'r 24. The output circuit 'of the vacuum tube amplifier 24 1s connected through a coil 25, indu'ctivcly `located with 4respect to the coil 26. By ymeans of the phase changer 22 the Wave *received bythe. antenna AB may be adjusted so that it is 180 out of phasewit-h the `componentV from the local ytransmitter ijn'i'pressed uponthe radio ieceivei'fi'om the antenna AR. The potentiometer 23 fmay be adjusted so that the balancing component picked 'up by the antenna `AB will be Ajust equal iny amplitude to the 'component derived from the antenna AR. In this manner the voice energy received from thelocal transmitting stv-ationmay be Vso completely balanced out that it will 'be much smaller in volume than the speech energy received from the distant transmitting antenna AT and,V in factfso small in amplitude 'that it Will not 'operate the an'iplitier-detector units.' to lock them up. A' similar b-al'ancingantenna AB', together with associa-ted 'phasechanging andV amplitude-controlling devices are provided in connection with the the circuitof Fig. 2 so that in thisrespect,`as Well as in the 'others previously noted, `the two Y circuits are' identical in construction and operation. i

Thev arrangement above described has the'A advantage that it enables two-Way operation with balance both at radio and voice 7 i ity 4would result if the voice-operated y ployed for transmission over the radio channels in opposite directions. The echo suppresser units employed as above described,

in addition yto performing `the Vfunctions 'al- V ready discussed, further act to suppress v echoes in the circuits in a manner similar to that in which echoes are suppressed in ordinary four-wire circuits. Since the circuit is operative only in one direction at a time and since singing isimpossible, the various gain controls in the system may be set np to their optimum positions for operation independently of each other and without regard to any singing difficulties.

The system above describedy has a further advantage in that it renders itinere diflicult for unauthorized persons to listen in onthe system by means oi radio apparatus. A person desiring to hear the conversations in both directions must provide liimselt With two receivers. If he only utilized one receiver and the receiver were made sutiiciently sensitive to receive the transmission from the distant radiating antenna, the receiving set would be overloaded'by tlie energy from the local antenna. It is there-- fore necessary to provide one receiving set with suitable directive receiving means and make it sutiiciently sensitive to receive from the distant antenna while discriminating against the local stations suiiciently to avoid serious noise. Another' set will then have to be provided for receiving from the local station, although in general it would not be necessary to balance this set with respect to transmission from the distant station as the level of the energy from the distant station would be so low with respect to .that from the local station as to be substantially f inaudible.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different from those illustrated without departing from the spirit and scope of the invention as defined in the following claims.

lVhat is claimed is:

l. In a radio system for two-way coinmunication between two distant localities comprising a radio transmitter and a radio receiver in each locality, four-wire connections for connecting said transmitter and receiver to a two-wire circuit in each locality, an echo suppressor unit a-ssociateifl with. each side of vthe four-wire circuit in each of the localities, and means controlled by said echo Suppressors operating in response to transmission in one direction to disable the complete system for transmission in the opposite direction.

2. In a radio system for two-way coinmunication between two distant localities comprising aradio transmitter and a radio receiver in each locality, four-wire connections for connecting said transmitter and receiver to a local two-Wire circuit in each locality, an echo suppressor unit associated with each side of the four-wire circuit in each of the localities, and means controlled by an echo suppressor unit in each locality for normally disabling one side of the local four-Wire connection in each locality to prevent circulation of energy in the local four- Wire connection, the echo Suppressors in each locality beingalso arranged to render the disabled side of the local four-wire connection operative and the other side of the connection inoperative in response to voice currents.

3. In a radio system for two-Way communication between two distant localities comprising aradio transmitter and a-radio receiver in each locality, four-wire connections for connecting said transmitter and receiver tota local two-wire circuit in each locality, an echo suppressor unit associated with each side of the four-wire connection in each of the localities, means controlled by an echo suppressor unit in one locality for normally disabling the west to east side of the local four-wire connection, means controlled by a corresponding echo suppressor unit in the other locality for normally disabling the east to west side of the local four-wire connection, and means controlled by each echo suppressor unit associated with one side of the local four-wire connection in each locality for disabling the opposite side of the four-Wire connection, said suppressors operating in response to the transmission in one direction to render the side of the local four-wire connection transmitting in that direction operative in one locality and for rendering the other side of the four-Wire connection in that locality inoperative, the Suppressors in the distant locality operating to maintain disabled the half of the local four-Wire connection transmitting in the opposite direction.

4. In a radio system for two-way coinmunication between two dist-ant localities comprising a radio transmitter and a radio receiver in each. locality, four-Wire connections ior connecting said transmitter and receiver to a local two-Wire circuit in each locality, an, echo suppressor unit associated with each side of the four-wire connection in each of the localities, means controlled by said echo suppressor units in each locality for preventing circulation ot energy over the local :tour-Wire connection in each locality when the system is idle, and means contiolled by said units in each locality in response to the transmission of energy in one direction for disabling the complete system for transmission in the opposite direction.

5. In a radio system `tor two-way communication between two distant localities comprising a radio transmitter and a radio receiver in each locality, four-wire` connections for connecting said transmitter and receiver to a local two-wire circuit in each locality, an echo suppressor unit associated with each side of the four-wire connection in each of the localities, means controlled by said echo suppressor units in eacli locality for preventing" circulation oi energy over tlielocal*tour-wire connection. in eacli'localn ity when the system is idleiiiieaiis controlled byv said units in each locality inl response to tlie transmission of` energy inoney direction Afor disabling tlie complete system for transmission inr tlie opposite direction` and iiie'aiisl associated with the lo lal radio receivei in eacli locality for balancing out energy radiated fromtlie local transmitter in order to `prevent noise currents 'from locking y lip one of'tlie eclio suppressoi's'iiitlie idle conf locality, an echo suppressor unit1 associated with eacli side of the four-'Wire connection ingang? in each oi"y the localities, means controlled by said echo suppressor units ineacli,localityvk Y for fpreventingj'circulation Aof enerb'y over the local four-Wire connection in eac'iVv A locality when tlie system is idle', means"controlledi by said units in each localityV in res onsey to the transmission of yenergy iny onei'rectioiiy for disabling;l the complete system ffoij transf associated Witli ythe radio receiver in each distant locality for impressing upon saidY radio receiver a/component equal in' ampli! tude and opposite in phase'to the energy,

component received from the localti'ansiriitf'A tei", thereby preventing noise ciirrentsraf` diated from the local ti'ansiiiitter froniloek ing up one of the echo suppressor units.

In testimoiiywvliereof, We have si nedour naines to tliis speciiication this '16t day'of:

December 1924.l Y

RALPH BOVN. HAROLD'S. HAMILTON,

30 mission in tlie opposite d irectionfand'means 

